Scavenging pump for two cycle internal combustion engines



Nov. 24, 1942. J. SHEPHERD 2,302,977

SCAVENGING PUMP FOR TWO CYCLE INTERNAL COMBUSTION ENGINES.

Filed Jan. 23', 1942 2 Sheets-Sheet 2 HAROLD E SHEP ERD INVENTOR.

BY'I GLQLLJL Patented Nov. 24, 1942 SCAVENGING PUMP FOR TWO CYCLE- INTERNAL CONIBUSTION ENGINES Harold F. Shepherd, La Grange, 111., assignor to Lufkin Foundry & Machine Company, Lufkin,

Tex.

Application January 23, 1942, Serial No. 427,891

4 Claims.

This invention is designed to improve the volumetric efificiency of the type of scavenge pump in which the displacement of the outer end of the power piston is utilized to aspirate and precompress the charge.

When the scavenge pump is independent of the main cylinder and piston its cylinder displacement may be made greater than that of the engine proper and low volumetric efiiciency may be compe a ed y a c re p i incr s in pumping cylinder dimensions.

In'the type of engine with which this invention is concerned high volumetric efficiency of the scavenge pump is of utmost importance sincethe displacement of the pump is strictly limited to the power cylinder bore and stroke.

One of the features of the present invention is to provide a pump chamber so as to put a charge of precompressed fuel in a receiver and to release such precompression mixture from the receiver to the combustion chamber and to then release the charge in the pump chamber directly into the explosion chamber so as to prevent the loss in efllcienc due to the re-expansion of this mixture in the pump chamber.

It is one of the objects of the invention therefore to provide a flash port leading directly from the pump chamber to the combustion chamber, which port is uncovered at the end of the combustion stroke for a short period of time so as to equalize the pressures in the pump chamber and the combustion chamber.

Another object of the invention is to provide a flash port in a two valved scavenging pump of a two cycle internal combustion engine.

Other and further objects of the invention will be readily apparent when the following description is considered in connection with the accompanying drawings, wherein:

Fig. l is a transverse vertical sectional view of the engine with the piston in its uppermost position.

Fig. 2 is a transverse sectional view taken on line 2-2 of Fig. 1.

Fig. 3 is a broken detailed sectional view of the Structure of Fig. 1 illustrating the piston in its lowermost position to uncover the flash port.

The engine includes a frame or body 2 having an elongated cylinder 3 therein. The top of the cylinder is closed by the head 4 which is cooled by the circulation of a cooling liquid thru the passages 5 and 6 and the piping 1.

Reciprocating in this cylinder 3 is the piston 8 which has the connecting rod 9 afiixed thereto. This piston is of particular construction having an elongated skirt H which extends about a barrier I I held in position by the rods l2 which project upwardly from the base 13. This barrier ll is in the form of a head which remains stationary so that the inside hollow portion of the piston 8 slides thereabout. As seen in Fig. l a precompression or pump chamber I is then formed inside of the piston 8 and above the barrier II. The skirt it! of the piston has a plurality of ports it therein which open into a passage I! in the frame, which is in reality a part of the precompression or pump chamber 15. This area I! serves as an inlet and an outlet for the precompression main pump chamber l5.

As seen in Fig. 1 when the piston 8 moves up and the chamber l5 enlarges from the size shown in Fig. 3 to the size shown in that of Fig. 1 it is apparent that there will be a suction thru the passage IT. This suction lifts up the flap valves Hi to draw a charge of air and fuel thru the carburetor Hi from the air inlet 20. On the reverse stroke as the piston 8 moves down, the explosive mixture in the chamber and the base I! will be compressed because the valves 98 have now closed. 'As this charge of explosive mixture is compressed, it will lift the outlet valves so as to admit the precompressed explosive mixture into the receiverzfi. This receiver is of considerable size, as is apparent from Fig. 2 and extends entirely around the cylinder 3, having the inlet ports 28 arranged circumferentially around one part of the cylinder. As the piston moves down to the position shown in Fig. 3, these ports 28 are uncovered, permitting a flow of explosive mixture into the combustion chamber 30, which is formed above the piston 8. It will be noted that the exhaust port 3| is slightly above the elevation of the inlet ports. 28, so that the combustion chamber 30 is open to exhaust slightly before it is open to intake. In this manner there will be a scavenging action in the chamber to discharge the spent gases and to permit their displacement by the incoming explosive mixture.

It seems obvious that when the compression chamber or scavenging pump arrangement at [5 assumes the proportions of Fig. 3, that then there is a considerable volume of explosive mixture which is under compression, namely, the area in the scavenging pump chamber at [5, as well as all of the mixture in the pump chamber l1 and also that area between the valves l8 and 25.

When the piston starts upwardly on the compression of the explosive mixture in the combustion chamber and begins to enlarge the pumping 5.1 chamber I5, the valves 25 instantly close and that -body of mixture in the chamber I1 begins to expand so that there would be a great deal of lost motion. In order to avoid this, a particular construction of the pump chamber I1 has been devised and this construction includes the upper inclined wall 35 which forms the top of the chamber IT. This wall extends upwardly as seen in Figs. 2 and 3 so as to form a port 31 which is uncovered as the piston 8 moves downwardly to its lowermost position. This port 31 is shown uncovered in Fig. 3. By uncovering this port for a very short period of time, it seems obvious that the fuel under the initial pressure in the chamber I I will now flash into the combustion chamber 30 so as to reduce the pressure in the chamber H.

In other words, the differential pressure which would otherwise remain in the pump chamber l I is permitted to escape into the combustion chamber. This predominant pressure, of course,

occurs in the passage II due to the provision of I valves 25.

As the piston moves upwardly a short distance, it will close the flash port 31 so that a suction will be instantly initiated in the scavenging pump chamber I5 and the pump chamber I I.

With the foregoing construction it seems obvious that a considerable volume of precompressed explosive mixture can be stored in the receiver 26 but that on each stroke the pressure in the chambers l1 and I5 can escape into the combustion chamber as the piston moves to the lower end of its stroke.

The usual details of construction have not been described specifically because it is believed that their construction and operation is well understood with engines of this type. The invention broadly contemplates a two-cycle internal combustion engine wherein a flash port is provided to permit the direct by-passing of a precompressed mixture from the precompression area.

What is claimed is:

l. A two cycle internal combustion engine of the scavenging pump type wherein the displacement of the outer end of the power piston is utilized as a pump to precompress the explosive mixture comprising, an engine frame, a cylinder therein, an elongated piston therein forming a combustion chamber, a stationary barrier within said piston forming a pump chamber, ports thru the wall of the piston, a passage in the frame for the surge of explosive fluid to and from said pump chamber thru said ports, a receiver area to receive the precompressed explosive mixture to be fed to the combustion chamber, a valve to prevent back flow from the receiver to the passage, said passage having a flash port extending to be uncovered by the piston at the end of its stroke to allow precompressed mixture remaining in the pump chamber to flash into the combustion chamber.

2. In a two cycle internal combustion engine having a scavenging pump in the piston cylinder, a receiver for precompressed fuel from such pump, ports to admit the fuel from the receiver to the combustion chamber, and means independent of said ports to conduct the precompressed fuel from said pump directly to the combustion chamber at the end of the precompression stroke.

3. In a two cycle internal combustion engine having a scavenging pump in the piston cylinder, a receiver for precompressed fuel from such pump, ports to admit the fuel from the receiver to the combustion chamber, and means independent of said ports to conduct the precompressed fuel from said pump directly to the combustion chamber at the end of the precompression stroke, said means comprising an area in the pump chamber to be uncovered by the piston.

4. A scavenging pump structure for two cycle internal combustion engines utilizing the displacement of the outer end of the power piston to precompress fuel, a pump chamber disposed at one side of the power cylinder, an inlet valve to said chamber, a discharge valve from said chamber, a receiver for the discharge expelled from the pump through said discharge valve, piston controlled ports in the cylinder wall for the passage of a charge of precompressed fuel from the receiver to the power cylinder, and a piston con trolled flash port leading directly from the pump chamber to the power cylinder.

HAROLD F. SHEPHERD. 

